Place the chicken breast between two sheets of parchment paper and pound with a meat mallet to an even half-inch thickness.
Place the chicken in a shallow dish and pour the buttermilk over it, ensuring it is fully submerged, and marinate for 30 minutes.
In a shallow bowl, whisk together the whole wheat flour, panko breadcrumbs, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, and dredge it thoroughly in the flour and panko mixture until well-coated.
Lightly spray the chicken on both sides with avocado oil and place it in the air fryer basket.
Air fry at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken is cooking, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a clean sandwich spread.
Lightly toast the whole wheat bun until the edges are crisp.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the romaine lettuce, crispy chicken, and dill pickles.